Electrostatic precipitation for particulate removal from gas streams is a technology that has successfully found application in systems ranging from domestic use at a few hundred cubic feet per minute to power plant systems with several million cubic feet per minute. In this process, dust laden gas flows between two electrodes maintained at a large electrical potential difference. The potential difference must be large enough to cause current flow between the electrodes by corona discharge. This in turn, causes charging of the particulate entrained in the gas flow.
Upon accepting a net charge, the particulate experiences an electrostatic force causing it to migrate to the collecting electrode where it is captured and removed from the gas stream. The forces holding the particulate on the collecting electrode are both electrical and adhesive/cohesive in nature.
This overall process can be conducted in either of two general geometries. The first is a "wire and tube" arrangement. In this system, the high tension electrode (wire or rod-like assembly) is located centrally in a tubular electrode which serves as the collection electrode. As gas flow occurs through the tube, particulate is forced to the outer tube where it is collected. In the more common "parallel plate" arrangement, the collecting electrodes are comprised of many parallel plates separated by a distance of typically 0.3 meter. Discharge electrodes consisting of wires or other corona enhancing shapes are placed between the plates.
In all geometries, the dust must periodically be removed from the collection electrode in order to maintain the electric field between the electrodes. The process by which electrodes are cleaned usually consist of a mechanical "rapping" of the plate or tube in such a fashion that the resultant vibrations and accelerations are adequate to shear the dust cake at the elecrode/dust cake interface causing the deposits to shed in large chunks and fall into collection hoppers below. Recent studies of the rapping process and the resulting propagation and distribution of separating forces have helped to quantify the design problems associated with this method of cleaning.
Recently there has been considerable effort expended in an attempt to extend electrostatic precipitation technology into the range of high temperature and pressure that is consistent with advanced coal conversion technologies such as pressurized fluid bed combustion and gasification. The primary motivation for this work is that in the typical operating environment of high temperature and pressure, very large electric fields can be sustained prior to electrical breakdown and spark over. This results in larger electrostatic forces on the particulate and increased migration velocities to the collection electrode. The end result is that by comparison to current standard electrostatic precipitator design, very compact and highly efficient precipitators can be developed for high temperature, high pressure applications.
A serious design problem arises in high temperature, high pressure precipitator applications when the electrode cleaning method is considered. The conventional method of mechanically rapping the collecting surface is apt to be difficult if not impossible because at high temperature, the properties of the electrode are greatly altered. Virtually all metals have very little mechanical strength at temperatures exceeding 800.degree.-900.degree. C. and are unlikely to be capable of surviving the traditional mechanical rapping required for cleaning. In addition, the transmission and distribution of vibrations will be greatly diminished in these materials at high temperature. A final problem to be overcome is the design and operation of a system that is capable of delivering the rapping action to the collection plates. Either the design of a system to operate from within the pressure boundary or to penetrate it will be difficult and costly.
It is an object of the present invention to provide an electrostatic precipitator which has means for cleaning dust deposits from the collecting electrode without the need to physically rap the electrode.
It is a further object of the present invention to provide an electrostatic precipitator that is usable under high temperature and high pressure conditions and is readily cleaned.